Linezolid, chemically N-[[(5S)-3-[3-fluoro-4-(4-morpholinyl)phenyl]-2-oxo-5-oxazolidinyl]methyl]acetamide is an antibacterial agent. Linezolid is represented by the following structure:

Linezolid and related compounds, processes for their preparation and their therapeutic uses were disclosed in U.S. Pat. No. 5,688,792. Processes for preparation of linezolid were also described in U.S. Pat. No. 5,837,870, WO 99/24393, J. Med. Chem. 39(3), 673-679, 1996 and Tetrahedron Lett., 40(26), 4855, 1999.
Linezolid is known to exhibit polymorphism and two crystalline forms are so far known. U.S. Pat. No. 6,559,305 and U.S. Pat. No. 6,444,813 addressed that the product obtained by the process described by J. Med. Chem. 39(3), 673-679, 1996 is form I and is characterized by having melting point of 181.5-182.5° C. and by IR spectrum having bands at 3284, 3092, 1753, 1728, 1649, 1565, 1519, 1447, 1435 cm−1. U.S. Pat. No. 6,559,305 claims crystalline form II characterized by IR spectrum having bands at 3364, 1748, 1675, 1537, 1517, 1445, 1410, 1401, 1358, 1329, 1287, 1274, 1253, 1237, 1221, 1145, 1130, 1123, 1116, 1078, 1066, 1049, 907, 852 and 758 cm−1 and powder X-ray diffraction spectrum having 2-theta values at 7.10, 9.54, 13.88, 14.23, 16.18, 16.79, 17.69, 19.41, 19.69, 19.93, 21.61, 22.39, 22.84, 23.52, 24.16, 25.28, 26.66, 27.01 and 27.77 degrees.
We have discovered a novel crystalline form (form III) of linezolid. The novel crystalline form of linezolid is consistently reproducible, does not have the tendency to convert to other forms and found to be thermally more stable than form I or form II. Furthermore, form III bulk solid is more compact and less electrostatic than form II and hence is more readily subjected to any treatment under the usual conditions of the pharmaceutical technology, in particular, of formulation on an industrial scale. Therapeutic uses of linezolid were disclosed in U.S. Pat. No. 5,688,792.
The object of the present invention is to provide a stable, consistently reproducible crystalline form of linezolid; processes for preparing it; and a pharmaceutical composition containing it.